Refrigerators operating at very low temperatures



April 9, 1968 H. LONDON 3,376,712

REFRIGERATORS OPERATING AT VERY LOW TEMPERATURES Filed Jan. 26, 1967 2 Sheets-Sheet 1 April 9, 1968 H. LONDON 3,376,712

REFRIGERA 'IORS OPERATING AT VERY LOW TEMPERATURES Filed Jan. 26, 1967 2 Sheets-Sheet 2 United States Patent Office aararrz Patented Apr. 9, 1968 3,376,712 REFRIGERATOR?) GEERATING AT VERY LQW TEMPERATURES Heinz London, Oxford, England, assignor to United Kingdom Atomic Energy Authority, London Engiand Filed Jan. 26, 1967, Ser. No. 612,037 Claims priority, application Great Britain, Mar. 16, 1966,

10 tllaims. (Cl. 62---467) ABSTRACT OF THE DISCLGSURE A refrigerator operating at temperatures well below 0.1" K. depends on taking a concentrated superfiuid solution of -helium3 in helium-4 (containing about 6% of helium-3) at a temperature of about 007 K. and diluting it reversably by the addition of helium-4 through a superleak, thereby doing external work.

Background of the invention The present invention relates to refrigerators operating at very low temperatures and having helium as the working fluid.

In our US. Patent Number 3,195,322 we described a helium refrigerator which depends upon the continuous dilution of a volume of liquid helium-3 with liquid helium-4, heat being absorbed from a sample to provide the heat of mixing. Such a refrigerator (hereinafter called the dilution refrigerator) has proved very satisfactory in practice and operates to reduce the temperature of the sample to below (11 K. and under suitable circumstances to 0.065 K. It will be known that this dilution refrigerator depends upon the fact that at these temperatures helium-3 and helium-4 separate into two phases, each containing only a few percent of the other isotope with the equilibrium concentration being dependent upon temperature. However, operation of the dilution refrigerator has shown that below a temperature of about (106 K. the equilibrium concentration of helium-3 in helium-4 remains substantially constant at about This has the thermodynamic consequence that the heat of mixing decreases rapidly with decreasing temperature and no further cooling is possible below that temperature.

It is an object of the present invention to provide a refrigerator capable of operating at a temperature below 0.06 K. In other words it is an object of the present invention to provide a refrigerator capable of providing useful cooling below the temperature at which the dilution refrigerator can be used.

Summary of the invention According to the present invention there is provided a refrigerator wherein a concentrated superfluid solution of helium-3 in helium-4 is diluted reversibly by the addition of helium-4 through a superleak, thereby doing ex ternal work.

Under these circumstances the solution is cooled, the temperature attained varying approximately as the twothirds power of the change in helium-3 concentration.

It should be explained that at the temperatures in question, a mixture of helium-3 and helium-4 behaves essentially as an ideal gas (actually a Fermi gas) wherein the helium-3 represents the gas and the helium-4 can be considered as being the containing space. The osmotic pressure of the helium-3 represents the gas pressure. It will be apparent that the helium-3 must be caused to expand reversibly and to do this a superleak has to be used to introduce and withdraw the helium-4 from the expansion chamber. It will be known that a superleak is a device which is substantially impermeable to helium- 3 but offers substantially no impedance to the flow of superfluid helium-4 and it will also be understood that at the temperatures in question the helium-4 will be superfiuid, whereas helium-3 does not exhibit the phenomenon of superfiuidity. Superleaks are described in more detail in the said prior patent.

The helium-4 is normally contained in a load container, so called because the helium-4 acts as a mechanical load opposing the expansion of the helium-3. Thus the mechanical work produced by the osmotic pressure of the helium-3 in the expansion chamber is delivered, through the superleak, to the load chamber where it is irreversibly dissipated. The load applied through the superleak can be adjusted at will by appropriately varying the temperature of the helium-4 in the load chamber, the fountain pressure of the helium-4 balancing the osmotic pressure of the helium-3 solution in the expansion chamber. This load chamber operates at a higher temperature than the expansion chamber and is thermally insulated therefrom, so that the heat resulting from the dissipation of the mechanical work can be absorbed without difficulty.

In fact the refrigerator of the present invention is best described by way of analogy with a steam engine. Thus the helium-3 solution fed to the expansion chamber can be considered as the high pressure steam, its osmotic pressure corresponding to the steam pressure. The free surface of the liquid in the expansion chamber represents the piston and the varying volume of liquid contained in the chamber corresponds to the varying space behind the piston. Whereas in the steam engine this space is occupied only by the expanding steam, it is here varied by the admission or withdrawal of the helium-4 passing through the superleak. Although the total pressure at the free surface is zero, it is made up of the positive partial pressure (osmotic pressure) of the helium-3 and the negative partial pressure of the helium-4. The helium- 4 in the superleak plays the role of a connecting rod operating under tension, the walls of the superleak acting as the gland. It transfers the work doneon the piston to the load container which acts as a brake. The cooling of the steam during the expansion corresponds to the cooling by reversible dilution in accordance with the present invention. Even the operational cycle of a conventional steam engine viz. 1) admission of steam at constant high pressure (here He-3 concentration) through an inlet valve until a fraction of the cylinder volunie is filled; (2) expansion (here dilution) to the full cylinder volume with valves closed; (3) exhaust of the steam at low pressure through an exhaust valve, is reproduced in the present invention.

In an alternative arrangement the brake is constituted H by a narrow capillary which has a resistance to the flow of superfluid helium-4; such a capillary would connect the superleak to a suitable source of reservoir of helium- 4, e.g. the boiler of the dilution refrigerator.

It is, of course, not essential that the refrigerator of the present invention be used in conjunction with the dilution refrigerator but such an arrangement shows certain advantages, chiefly in that it provides a ready supply of cold feed solution and a means of disposal of the helium-3 from the diluted solution. The cold feed solution of the present invention is described as concentrated solution of helium-3 in superfluid helium-4 and for effi ciency in operation the concentration should be as high as possible. However it is not possible to use substantially pure helium-3. Thus the feed solution of the present invention is the solution which, in the rior patent, is described as the helium-4 phase. The feed solution is n) conveniently obtained by placing substantially pure helium-3 in contact with super-fluid helium-4 and using the saturated solution of helium-3 in helium-4 thus obtained.

Brief description the drawings In order that the present invention may more readily be understood, two embodiments of the same will now be described by way of example and with reference to the accompanying drawings, wherein:

FIGURE 1 is a diagrammatic representation of a first embodiment of the invention, and

FIGURE 2 is a similar representation of a second embodiment.

The refrigerator illustrated in the drawings is developed from FIGURE 2 of the said prior patent for simplicity.

Description of the preferred embodiments Referring now specifically to FIGURE 1 of the drawings, a cooling chamber 1 carries a container 2 for an article to berefrigerated and is connected at its lower end through a U-tube 3 to the bottom of a boiler 4 heated by an electric heater 5. A vapour outlet duct 6 leads from the boiler 4- to an external pumping system by way of a pipe 7. In order to retain the helium-4 in the boiler 4 against its tendency to creep up the outlet duct 6, a diaphragm 8 is provided in the duct immediately above the boiler 4 and has an aperture of the minimum size necessary to pass the helium-3 vapour. In the external pumping system the vapour from the boiler 4 (essentially helium- 3) is recompressed from its original pressure of about 0.002 mm. Hg to a pressure of about 30 mm. Hg at which pressure it can he condensed in a pocket 9. The external pumping system is not shown, but should have a capacity of the order of 1,800 litres per second at 0.001 mm. Hg.

In order to provide for this condensation, the pocket 9 is cooled with liquid helium-4, derived from an external source and evaporated under reduced pressure to give a temperature of about 13 K. This helium is contained in a flask 10 which is provided with suitable filing means and vapour extraction means (not shown).

The helium-3 condensed in the pocket 9 may be admitted by a valve 11 to a tube 12 which passes in heat exchange with the boiler 4 and in counter-current heat exchange withthe U-tube 3 to a phase separator 13 (which has the same position in the circuit as the working chamber of the prior patent). During operation the phase separator 13 contains the phase boundary through which helium-3 passes from the upper concentrated phase to the lower dilute phase. However, as explained, the present invention is useful at temperatures where no further cooling takes place on the passage of helium-3 through this boundary and consequently the article to be refrigerated is no longer located at the phase boundary. The phase separator now acts as the source of a concentrated solution of helium-3 in helium-4, the helium-3 in this solution being passed by a tube 14 and inlet valve 15 to a narrow tube 16 leading to an expansion chamber 17, the bottom of which is located above the level (indicated at 4') of the liquid in the boiler 4. The narrow tube 16 has a side tube 18 which is below the level 4' of liquid in the boiler 4 and which leads to an outlet valve 19 and thence by a pipe 20 to the cooling chamber 1.

The expansion chamber 17 is also connected by a superleak 21 and pipe 22 to a load chamber 23 which contains a heater 24. The load chamber 23 is in thermal contact with the boiler 4 through a heat switch 25 which comprises two members 26 and 27 of high thermal conductivity linked by a superconducting member 28. This link is surrounded by a solenoid 29 conveniently wound with superconducting wire of a critical magnetic field above that of the link 28). When the solenoid 29 is energised, the superconducting link 28 changes into the normal state in which its thermal conductivity is high.

Thus the switch is closed and the temperature of the load chamber 23 falls to that of the boiler 4. On the other hand, when the link 28 is superconducting, its thermal conductivity is low, the heat switch is "open and the temperature of the load chamber 23 can be raised by the heater 24. It may be possible to dispense with the heat switch by giving the heat link an appropriate heat resist ance and controlling the temperature of 23 by the heater 24 only.

The whole system below the pocket 9 is contained in a jacket 30 immersed inthe flask 10 and evacuated through a tube 31 to provide thermal insulation. I H

In operation, a mixture of helium-3 and helium-41s initially condensed into the pocket 9 and admitted by the valves 11, 15 and 19 until it substantially fills the cool ing chamber 1 and enters the boiler 4; As the bottom of the expansion chamber 17 is above the level of liquid in the boiler 4, this chamber will remain empty.- With the heat switch 25 open and a small amount of heat being supplied by the heater 24, the load chamber 23 will fill itself with pure helium-4 by the fountain effect acting through the superleak 21. The pump connected to the pipe 7 is switched on and helium-3 is withdrawn from the system and is recondensed in the pocket 9. As a result the helium-3 concentration in the boiler 4 decreases, since it evaporates at a much faster rate than the helium-4, and, at a certain low concentration of helium-3, the liquid in the boiler 4 becomes superfluid and this causes a how of helium-3 relative to the helium-4 in the U-tube 3 and the spreading of the superfiuid region of low helium-3 concentration towards the cooling chamber, valve 19, side tube 18, valve 15, tube 14 and phase separator 13, cooling it in the process. As the concentration gradient in the phase separator rises and the temperature falls, phase separation sets in and a phase bound ary is formed. The amounts of helium-3 and helium-4 originally condensed into the system are adjusted so that when phase separation takes place the load chamber is filled with pure helium-4, the tube 12 and a small part of the phase separator 13 are filled with concentrated helium 3, whilst the remainder of the system is filled with a dilute solution of helium-3 in hclium 4. The volume of the phase separator 13 is so chosen that variations in phase bound ary, due to variations in helium-3 concentration, take place in this chamber. As the temperature continues to fall in the boiler 4, the heater 5 is operated to maintain the temperature in the boiler 4 at about 0.6 K.

Finally a steady state is reached in which the helium-3 in the tube 12 is approximately concentrated whilst the helium-4 in the space from the phase boundary to the outlet of the cooling chamber 1 contains about 6% of heliurn-3 at a temperature of 0.07 K. and the liquid in the boiler 4 contains about 1% of helium-3, the osmotic pressure in the boiler and cooling chamber 1 being ap proximately equal. So far the system has been operating as a dilution refrigerator and the temperature in the cooling chamber 1 will fall until it reaches the lowest value obtainable by utilising the heat of dilution. At this stage the valve 19 is closed and the refrigerator of the pres ent invention is put into operation.

In order to operate the refrigerator of the present invention, the heater 24 is turned off and the heat switch 25 closed by energising the solenoid 29. As the temperature of the liquid helium-4 in the load chamber 23 falls, the fountain pressure becomes less than the osmotic pres,- sure of the helium-3 in the tube 16 and liquid helium-4T- begins to flow from the load chamber 23 through the superleak 21 into the tube 16 and from thence into the: expansion chamber 17, this being possible as the valve: 19 is closed. Simultaneously helium-3 flows across the phase boundary in the phase separator 13 so that the concentration of helium-3 in the helium-4 in the chamber 17 is only slightly less than that in the phase separatorchamber 17 has been filled in this way, the valve is closed, thus completing the first stage of the steam cycle analogue.

In the second stage of the cycle, helium-4 continues to flow into the expansion chamber 17, causing the concentration of the helium-3 to fall as the valve 15 is closed. As a consequence, the temperature and the osmotic pressure of the helium-3 in the expansion chamber 17 fall. The fountain pressure on the helium-4 in the load chamber 23 will also fall and therefore the rate of flow of helium-4 is controlled by the rate of cooling of the load chamber 23 across the closed heat switch 25. The second stage of the cycle is completed when the expansion chamber 17 is full. The helium-3 concentration in the expansion chamber 17 falls to about one eighth of its original value (e.g. to about 0.75%) and in the absence of heat leaks the temperature, in accordance with the two-thirds law, will have fallen to about one quarter of its original value (e.g. to about 0.()2 K.). However during the time that the valve 19 has been closed, the boiler 4 has continued to operate to lower the concentration of helium-3 and consequently lower the temperature of the cooling chamber 1.

The third stage in the cycle can begin when the concentration of helium-3 in the cooling chamber 1 is below that in the expansion chamber 17 and during the first cycle of operations it may be necessary to wait for this to take place. The third stage is initiated by opening the valve 19 and also opening the heat switch and energising the heater 24. As a result helium-4 will flow back to the load chamber 23 and helium-3 (due to the concentration gradient) will fiow from the expansion chamber 17 via the cooling chamber 1 and U-tube 3 to the boiler 4. If these two flow rates are not exactly balanced, helium-4 will tend to flow through valve 19 to stabilise the helium-3 concentration in the expansion chamber 17. The third stage is completed when the expansion chamber 17 is empty and the load chamber 23 is full; the valve 19 is then closed.

During the fourth and last stage of the cycle, the temperature of the helium-4 in the load chamber 23 is raised until the fountain pressure is high enough for the first stage to begin again.

Subsequent cycles are the same as that outlined above, except that there is no need to wait for a fall in the concentration of helium-3 in the cooling chamber 1, as this condition will automatically be satisfied.

To summarise: the valve 15 is open only in the first stage of the cycle; the valve 19 is open only in the third stage of the cycle; the heat switch 25 effects cooling of the chamber 23 only in the first and second stages of the cycle; whilst the heater 24 effects heating of the chamber 23 only during the third and fourth stages of the cycle. The volume of helium-4 in the chamber 23 reaches a minimum at the end of the second stage of the cycle whilst the chamber 17 is empty during the fourth stage of the cycle.

A second embodiment of the invention is illustrated in FIGURE 2 and is simpler in construction and operation but may not be so efficient. This second embodiment di ffers in that the load chamber 23 and heat switch 25 are replaced by a capillary tube 32 which connects the superleak 21 to the boiler 4, being in good thermal contact with the latter. The system operates as a dilution refrigerator in precisely the same way as that of FIGURE 1.

During the first stage of the cycle the valve 19 is closed, as before, whereupon the concentration of helium-3 in the boiler 4, U-tube 3 and cooling chamber 1 will fall, but this concentration will remain constant in the phase separator 13 and tube 16. Consequently the osmotic pressure of the helium-3 in the tube 16 will be higher than that in the boiler 4 and helium-4 will be drawn through the capillary 32 and superleak 21 into the tube 16 and expansion chamber 17. As before, valve 15 is closed when the expansion chamber 17 is about one eighth full.

During the second stage of the cycle, helium-4 continues to flow from the boiler so long as the osmotic pressure in the expansion chamber 17 (decreasing due to the dilution and cooling of the helium-3) exceeds the osmotic pressure in the boiler 4 by the hydrostatic head due to the difierence inliquid levels in the boiler and expansion chamber. This condition, which is not imposed in the construction of FIGURE 1, restricts the degree of dilution obtainable and the end of the second stage is reached when the liquid level in the expansion chamber ceases to rise.

During the third stage, the valve 19 is opened and all the helium-3 and most of helium-4 will flow into the boiler 4 via the tube 16, pipe 18, pipe 20, chamber 1 and U- tube 3 under the influence of the hydrostatic head in the chamber 17. Only a small proportion of the helium-4 will return directly to the boiler via the superleak 21 as the capillary 32 offers a high resistance to the flow of helium-4.

When the chamber 17 is empty, the fourth stage may begin by closing valve 19 and open-ing valve 15. During this fourth stage the concentration of helium-3 above the valve 15 rises quickly to that in the phase separator 13. The fourth stage mergesinto the first stage of the next cycle when the helium-4 starts to flow through the superleak 21. However, the helium-4 only flows slowly because of the load imposed by the capillary 32.

I claim:

1. A helium refrigerator comprising a load container, an expansion chamber, a superleak connecting the load container and the expansion chamber, means to introduce a supe-rfiuid solution of helium-3 in helium-4 partially to fill the expansion chamber, and means to introduce helium-4 from the load container into the expansion chamber via the superleak and to withdraw it by the same route, thereby reversibly to dilute the solution of helium-3 and do external Work.

2. The refrigerator of claim 1, including means to vary the temperature of the helium-4 in the load container.

3. The refrigerator of claim 2, wherein the load conainer is thermally insulated from the expansion chamber.

4. The refrigerator of claim 1, having a dilution refrigerator in association therewith.

5. The refrigerator of claim 4, including a heat switch coupling the load container with a boiler of the dilution refrigerator.

6. The refrigerator of claim 5, wherein the heat switch includes an electrically su-perconducing member and a solenoid adapted to provide a magnetic field of sufficient intensity to destroy the superconductivity of the saidmemher.

7. The refrigerator of claim 4, including a capillary tube acting as the load container and connecting a boiler of the dilution refrigerator to the superleak.

8. The refrigerator of claim 1 including a cooling chamber having means to receive the article to be cooled and connected to the expansion chamber by a conduit including a valve.

9. A helium refrigerator comprising a source of a saturated superfiuid solution of helium-3 in helium-4 (13), an expansion chamber (17), means (14, 15, 16) to feed the saturated solution of helium-3 in helium-4 into the expansion chamber, a load container (23) containing helium-4, a superleak (21) connecting the load container to the expansion chamber, and means to feed helium-4 from the load container into the expansion chamber and from the expansion chamber back to the load container, whereby the solution of helium-3 in the expansion chamber is reversably diluted.

10. A helium refrigerator comprising a phase separator (13), a boiler (4), conduit means (3) to provide a superfiuid helium-4 link between said phase separator and said boiler, means (9, 11, 12) to pass liquid helium-3 to the phase separator to mix therein with superfluid helium-4 whereby heat is extracted, heating means (5) associated with said boiler to evaporate helium-3 from the helium-4 therein, means (6) to remove the helium-3 vapour from the boiler, an expansion chamber (17), a load chamber (23, 32), a swperleak (21) between the load chamber and the expansion chamber, means (14, 15, 16) to pass a solution of helium-3 in helium-4 into the expansion chamber, and means to cause helium-4 to flow from the load chamber into the expansion chamber reversably to dilute the solution therein and do external work.

References Cited UNITED STATES PATENTS 3,125,863 3/1954 Hood 62-77 X 3,195,322 7/1965 London 62-467 3,269,137 8/1966 Hood 62403 3,313,117 4/1967 Hood et a1 625 14 LLOYD L. KING, Primary Examiner. 

